Summary: Group B Streptococci (GBS) are the leading cause of neonatal sepsis and meningitis. Maternal IgG antibodies to the GBS polysaccharide (PS) protect the neonate from invasive GBS disease. Such antibodies are deficient in mothers of infants who develop GBS disease. A case control study was designed by NICHD to identify amounts of antibody needed for protection against type Ia. Paired maternal and cord sera from different regions of the United States were analyzed in our laboratory for presence of naturally occurring GBS type Ia PS antibodies. Statistical analysis of the level of antibodies in maternal and cord sera indicated that 5 ug/ml was 88% effective at preventing early onset GBS Ia disease. The corresponding protective cord serum level was 4 ug/ml. Measurements of antibody concentrations for GBS type III have proven to be more challenging. We have established a GBS PS purification method whereby GBS culture supernatant is used as the starting material to purify native GBS type III PS with no harsh treatment, while other investigators have used the bacterial pellet and NaOH to remove the group antigen followed by reacetylation. Earlier work in our laboratory has shown that conjugation of the GBS III PS to albumin changes some of the PS epitopes. We have now investigated effects of different conjugation methods on the epitopes and immunological reactivity of the GBS type III PS using ELISA and inhibition ELISA. ELISA antigens used were native and chemically modified GBS III PS (periodate or cyanylation), and PS from S. pneumoneae type 14, which is structurally the same as the GBS III PS except without terminal side chain sialic acid residues. The antibody samples used included an immune human serum against native GBS III PS, and monoclonal antibodies (Mab) to the GBS III and pneumococcal type 14 PSs. The GBS III Mabs bound strongly to native and modified GBS III PS, but not to the pneumococcal type 14 PS. Such binding was readily inhibited by native and modified GBS III PS, but not by the pneumococcal type 14 PS. All pneumococcal type 14 Mabs bound to the type 14 PS, but not to the native GBS III PS; some bound strongly to periodate treated GBS III PS, while others bound no GBS III antigen. Results of in vitro opsonophagocytosis experiments indicated that the pneumococcal Mabs cross-reactive with a periodate induced GBS III epitope were not opsonic for GBS type III. Thus, the native GBS III PS possesses few epitopes cross-reactive with the pneumococcal type 14 PS, chemical modification increases such epitopes and antibodies to these epitopes are not opsonic for GBS III.